The present invention relates to the reclaiming of green sand used in metal casting operations.
Green sand is a mixture of sand, clay and water used in the formation of molds into which molten metals are poured and allowed to cool sufficiently to permit a molded metal object to be removed therefrom without injury. Typically, green sand has a moisture content of between 3% and 4% and a clay content of between 5% and 12% by weight of sand with the moisture and clay being essentially uniformly distributed throughout the body of the mold prior to the introduction of metal.
After the introduction of molten metal into a mold formed of green sand, the metal slowly solidifies giving off heat to the mold, which serves to heat the sand and partially evaporate moisture, which escapes from the mold into the environment as water vapor, with heating effects within the mold being progressively less in directions away from the molding cavity. As by way of illustration, heating of the mold in an inner region thereof immediately adjacent the mold cavity will be sufficient to both evaporate all free moisture present and heat the binder surrounding the sand grains above a critical temperature at which it loses its water of hydration and becomes deactivated or "dead burned", such that it cannot thereafter reabsorb water for purposes of binding sand grains together during a subsequent molding operation. In a next outer region, the heat imparted to the mold will be sufficient to effect drying of the binder, while permitting same to be reactivated by the subsequent addition of water, and finally in more remote regions heating will be such that only a portion of the moisture content may be lost.
It is generally accepted that the types of clays typically used as binders in foundry applications become fully deactivated when exposed to temperatures exceeding about 600.degree. C. and can be fully reactivated by the subsequent addition of water when exposed to temperatures less than about 300.degree. C. Clay binders exposed to temperatures intermediate 300.degree. C. and 600.degree. C. are partially deactivated or damaged for subsequent molding purposes, but can be partially reactivated.
After solidification of a metal casting, the casting is separated from the molding sand by a shakeout device and the sand collected for further use. During shakeout, differently heated portions of molding sand are randomly mixed, such that it is not possible to remove sand bearing deactivated binder from remaining portions of the used molding sand having binder coatings, which remain activated or are at least partially subject to reactivation. Reuse of such molding sand without the removal of deactivated binder and the addition of new binder, would eventually render the molding sand unfit for purposes of making molds. To prevent this, it has been common practice to add at the completion of each molding cycle a predetermined percentage of new sand and clay and to remove an identical amount of old molding sand from the sand system. An obvious disadvantage of this procedure, is that a part of the discarded molding sand will comprise sand and active binder or binder subject to reactivation and a part of the molding sand retained for reuse will contain deactivated binder. Moreover, the raising costs associated with obtaining new sand and binder to replace discarded moldingsand and with disposal of the discarded molding sand in landfill sites renders this accepted procedure uneconomical in many regions of the world.
A variation or modification of the above described practice is proposed in an article by D. Lawson and Applicant entitled, "Reclaimed Shakeout Sand As New Sand Substitute in Green Sand Molding Lines", appearing in AFS Transaction 84-28, wherein sand processed in a reclaimer of the general type described in pending U.S. patent applicantion Ser. No. 720,129, filed Apr. 5, 1985, but without the addition of heat, is substituted for upwards of 80% of an otherwise required new sand addition. In this procedure, a batch of old molding sand, which would have previously been withdrawn from the sand system and discarded, is subjected to attrition in the presence of a vacuum exhaust for a period of time determined to maximize the amount of calcined binder separated from the sand grains and removed from the batch, while minimizing the amount of activated binder so separated and removed. The processed batch, together with a small amount of new sand and binder, are then mixed wtih unprocessed molding sand from which the batch had originally been withdrawn and then sufficient water is added to the resultant sand system in a mulling device, as required to increase its moisture content to a level suitable for a subsequent molding operation.
Certain problems are inherent in processing a sand system in accordance with the above described practice, including its modified form, due to the non-uniformity of moisture and thermal conditions existing both within individual molds, particularly when molds have a large sand to metal weight ratio, and between molds served by the sand system, which occurs under actual foundry conditions when molding sand is shaken from both hot molds into which molten metal had been poured and cold molds, which for some reason, such as damage to the mold, have not been charged with molten metal.
As regards moisture content, sand from cold molds, as well as sand from remote regions of hot molds having a large sand to metal weight ratio, have or may have a moisture content exceeding about 1.5% at which the molding sand tends to be "sticky" and will blind fine 20 mesh screens preferred to be employed to remove foreign objects and agglomerates from molding sand prior to its reuse. As by way of example of this problem, a study conducted in a steel foundry determined that the mean moisture content of 48 samples of molding sand taken over a 30-day period was about 0.85%, which is sufficiently low to guarantee that molding sand will be free flowing through a 20 mesh screen. However, four occurrences were noted during this period in which moisture content exceeded 1.5%, such that blinding of a 20 mesh screen would occur, and thus disrupt the sand reclaiming process.
As regards temperature variations present in a sand system after shakeout, it is known that the "compactability" for mold forming purposes of molding sand having a correct moisture content falls off sharply above about 120.degree. F. In the above study, the temperature of the samples fluctuated over a wide range with a mean temperature determined to be about 146.degree. F. The most simple approach to the problem of reducing the temperature of molding sand is that of storing the sand in suitable storage bins or hoppers, but this approach is extremely time consuming and requires substantial investment in storage facilities. It has also been proposed to reduce the temperature of molding sand by passing relatively cool air therethrough and by the addition of water. While these latter approaches are employed, difficulties may nonetheless be encountered, due to wide temperature variations within the sand system, which may result in the removal of an excessive amount of fines including active binder and the introduction of excessive moisture tending to render portions of the sand system non-free flowing for screening purposes, respectively.
An alternative procedure is to process the whole or part of a sand system under heat and attrition conditions sufficient to effect complete removal of all binder and moisture from the sand system in order to produce "clean", free flowing sand to which new binder and water is then added. However, in the absence of controlled thermal processing of the sand system in accordance with the teachings of the above-mentioned patent application, costs involved in heating used molding sand and subsequent cooling of "clean" sand is economically prohibitive.